The present disclosure relates to oversampling A/D converters, and more particularly to continuous-time delta sigma A/D converters.
Oversampling A/D converters are widely used for front ends of communication devices and conversion of audio signals, and are essential circuit techniques for present communication, video and audio signal processing circuits. As a type of oversampling A/D converters, there are continuous-time delta sigma A/D converters (CTDS-ADCs) including continuous-time filters such as integrators (see, for example, Richard Schreier and Bo Bang, Delta-Sigma Modulators Employing Continuous-Time Circuitry, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 43, NO. 4, APRIL 1996, and Xuefeng Chen et al., A 18 mW CT ΔΣ Modulator with 25 MHz Bandwidth for Next Generation Wireless Applications, IEEE 2007 Custom Integrated Circuits Conference, 2007).
In a conventional CTDS-ADC, an input signal passes through a number n of cascade-coupled integrators and is quantized by a quantizer. An output of the quantizer is converted to current signals by a number n of D/A converters, and then fed back to the respective number n of integrators. Since CTDS-ADCs do not include any switch in an analog circuit, voltages can be lowered. Moreover, CTDS-ADCs do not require any prefilter which is usually needed when using a sampling filter. In these respects, CTDS-ADCs are suited for application of communication systems, and the application has been increasingly researched and developed.
The accuracy of A/D conversion of a conventional CTDS-ADC largely depends on linear characteristics, a dynamic range, etc. of continuous-time filters through which an input signal passes. Thus, some converters are modified by feeding forward an input signal to an input of a quantizer, and feeding back an output of the quantizer to an input of the continuous-time filters so that the input signal is removed at a stage preceding the continuous-time filters, and only quantized noise passes through the continuous-time filters (see, for example, Paulo G. R. Silva, Lucien J. Breems, Kofi A. A. Makinwa, Raf Roovers, Johan H. Huijsing, An 118 dB DR CT IF-to-Baseband ΣΔ Modulator for AM/FM/IBOC Radio Receivers, ISSCC 2006/SESSION 3/OVERSAMPLING ADCs/3.3, Feb. 6, 2006). With this configuration, linear characteristics and a dynamic range required for the continuous-time filters are largely reduced, thereby simplifying the circuit configuration and reducing power consumption. On the other hand, the technique for improving operational stability of a CTDS-ADC by feeding back an output of a quantizer to an input of the quantizer is also known (see, for example, G. Mitteregger, C. Ebner, S. Mechnig, T. Blon, C. Holuigue, E. Romani, A. Melodia, V. Melini, A 14b 20 mW 640 MHz CMOS CT ΔΣ ADC with 20 MHz Signal Bandwidth and 12b ENOB, ISSCC 2006/SESSION 3/OVERSAMPLING ADCs/3.1, Feb. 6, 2006).